Scanning tunnelling microscopy and spectroscopy of quantum materials
Abstract
Scanning tunnelling microscopy and spectroscopy (STM/STS) of heavy fermion materials
and superconductors has enabled the high energy and momentum resolution probing
of their electronic and many-body interactions. Heavy fermion superconductivity and
its links to unconventional pairing mechanisms remains not fully understood; and the
potential role for topology in governing the surface properties of these materials is an
exciting new topic in condensed matter physics.
In this thesis I will present STM/STS investigations into four quantum materials with
heavy fermion and/or superconducting properties. In UPt3 the heavy fermion behaviour
is investigated, resulting in the visualisation of the Kondo lattice. Signatures of inelastic
tunnelling were seen, consistent with known phonon excitations; as well as a small
gap-like structure above the Fermi level. No superconducting gap was detected at the
surface of UPt3 and quasiparticle interference (QPI) revealed light bands at the Fermi
level. The spectroscopy of PdTe2 is then investigated, revealing sharp peaks in conductance
consistent with band extrema seen in ARPES; as well as conventional type-II BCS
superconductivity. Strong QPI signatures were revealed to be consistent with scattering
of quasiparticles from the topological surface states seen in ARPES, with a chiral spin
texture dictating the q-space structure of the QPI imaging. The superconductivity at
high magnetic eld in FeSe was then investigated, revealing a two-band character gap
structure with signi cant anisotropy. The superconducting gap persisted up to 14 T;
suggesting that FeSe remains a superconductor up to even higher magnetic elds. Finally
QPI measurements on SmB6 were compared to a bulk tight binding model, which
demonstrated that the surface band structure and QPI is well described by a bulk model
and in particular f-states and their hybridisation. Evidence for topological properties
are not obvious but in-gap conduction is seen, therefore providing some metallic surface
states of some kind.
Type
Thesis, PhD Doctor of Philosophy
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